Hello Radek Šimčík

@api.model
def _default_company_id(self):
if self._context.get('default_project_id'):

company_id = fields.Many2one('res.company', string='Company', required=True, default=_default_company_id)

It seems to me that the issue happens only when the task was created via email.

Please check.
In Project.task have company field check when your task created from Email, It populated or not .

If not populated then you have to passed company_id field in context via mail.
like this.
'context': dict(
self.env.context,
defualt_company_id=company.id,
),

Thanks & Regards,

Email: odoo@aktivsoftware.com

Skype: kalpeshmaheshwari